The present invention relates to the field of transmission systems. More particularly, the present invention relates to wireless transmission systems.
Orthogonal frequency division multiplexing (OFDM) systems are employed or proposed in many commercial wireless data transmission systems, for example, cellular telephones, digital audio broadcasting (DAB), and high-definition television (HDTV). These applications require the transmission of data at very high rates ranging from a few hundred kb/s to a few Mb/s.
OFDM systems are well known in the art of signal transmission. U.S. Pat. No. 5,610,908 to Shelswell et al., entitled xe2x80x9cDigital Signal Transmission System Using Frequency Division Multiplex,xe2x80x9d discloses an OFDM system for signal transmission, and is incorporated herein by reference.
In OFDM systems, a high rate serial data stream is converted to several parallel low rate data streams so that the symbol duration of each of the data symbols transmitted in each of the parallel streams is very large in comparison to the expected channel delay spread. The channel delay spread arises from multipath scattering. Multipath scattering occurs when a transmitter transmits a data pulse and because of the data pulse reflecting off of natural and man-made objects, the data pulse becomes many pulses which arrive at the receiver at different times. The difference in time between the first and last of the multipath data pulses is the channel delay spread. Each one of the parallel data streams are transmitted on a sub-carrier frequency such that the parallel data streams are orthogonal to each other. Since the sub-carrier frequencies of the parallel streams are less than the frequency of the serial data stream, the effects of delay spread are greatly reduced. Hence, unlike in single carrier transmission systems, equalization is not a difficult task in OFDM systems.
In the frequency domain, the bandwidth of each of the parallel data streams in the OFDM system is very small in comparison to the coherence bandwidth of the channel. The coherence bandwidth is the frequency range over which two frequency components will have a strong potential for amplitude correlation and is inversely related to the delay spread. Strong amplitude correlation will result in portions of the channel being xe2x80x9cflatxe2x80x9d (i.e., having a constant amplitude and linear phase response). Portions where signals cancel each other out are referred to as flat fading regions. Large flat fading regions result in poor signal reception at the receiver. Since the total transmission bandwidth is typically several times larger than the channel coherence bandwidth, some of the parallel data streams within the total transmission bandwidth will be subjected to flat fading regions. The flat fading is referred to as Rayleigh fading. Rayleigh fading arises from the transmitted signal being reflected off of different objects and arriving at a receiver at different times. These multipath signals create standing waves at the receiver and result in poor signal reception. The bit error rate (BER) of a signal tends to increase as a function of Rayleigh fading. Flat fading regions due to multipath signals creating standing waves will also arise in multiple transmitter systems where each transmitter is transmitting the same signal.
FIG. 1A illustrates a prior art orthogonal frequency division multiplexing (OFDM) system transmitter 10 for use in a single frequency network (SFN) of transmitters. A SFN comprises more than one transmitters transmitting the same OFDM signal in order to increase broadcast coverage. In transmitter 10, data bits 12 are initially received at the transmitter 10. The data bits 12 are then encoded using a convolutional coder 14 which encodes the data bits 12 into a continuous bit stream (1s and 0s) in a known manner using an error protection code. After encoding, the continuous bit stream is phase shift keyed using a phase shift keying PSK modulator 16 to obtain a stream of data symbols.
The data symbols are then frequency interleaved using frequency interleaver 18 to randomize the data. Interleaving is a data communication technique used in conjunction with error protection codes to reduce errors. In the interleaving process, coded data symbols are reordered before transmission in such a manner than any two successive coded data symbols are separated in the transmitting sequence. Upon reception, the interleaved coded data symbols can be reordered in their original sequence, thus effectively spreading or randomizing the errors in time to enable more complete correction by a random error protection code.
The symbols are then differentially encoded using differential coder 20. Differential encoding assists in making the data signal insensitive to phase distortion. The differentially encoded complex symbols are then fed into an inverse fast Fourier transformer 22. The inverse fast Fourier transformer 22 shifts the data symbols from the frequency domain to the time domain for data transmission. The signal generated by the inverse fast Fourier transformer 22 is an orthogonal signal in the time domain suited for data transmission. The time domain signal is then transmitted by antenna 24.
FIG. 1B illustrates a prior art orthogonal frequency division multiplexing (OFDM) system receiver 30 for receiving signals from a single frequency network (SFN) of transmitters. In the receiver 30, an OFDM signal is received at a receiver circuit 34 through antenna 32. The OFDM signal is then transformed from the time domain to the frequency domain by fast Fourier transform (FFT) 36. The transformed signal is then differentially decoded and de-interleaved by differential decoder 38 and de-interleaver 40, respectively. Next the signal is passed through a PSK de-modulator 42 to convert the complex symbols back into a bit stream. The bit stream is then decoded by convolutional decoder 44 to obtain the original data bits 12 from transmitter 10.
To improve signal reception, many OFDM systems, such as digital audio broadcasting (DAB) systems, use a single frequency network (SFN) of transmitters broadcasting identical signals in the same frequency band with the transmitters synchronized in time. The signals from these different transmitters appear as multi-path energy at the receiver and may improve signal reception because of frequency diversity.
Frequency diversity schemes are used to develop information from several signals transmitted over independently fading paths. The independently fading paths are combined to reduce the effects of flat fading regions. This scheme minimizes the effects of flat fading regions since flat fading regions seldom occur simultaneously during the same time interval on two or more paths.
The transmit power and the distance between the transmitters in a SFN are carefully designed so that the signal paths from the different transmitters arrive within a specified guard interval of an OFDM frame so that intersymbol interference from adjacent OFDM frames can be avoided. In a SFN, depending upon the path delays, the combination of signal energies from two or more transmit stations may lead to destructive addition of some frequencies of the signal at the receiver. In channels that have very small delay spreads this destructive addition of the signals will give rise to very poor performance gains.
In a multipath environment, if the signals from the transmit stations have very small delay spreads, frequency diversity is unavailable. To illustrate the effects of small delay spreads consider the following example for a digital audio broadcasting DAB system in a single frequency OFDM network. Consider two stations transmitting a 4 MHz wide signal centered at a frequency of 2.4 GHz. If the receiver has only line-of-sight reception from both transmit stations and is located equi-distant from the two transmit stations, at this location, as long as the path delay spread is not large (i.e.,  less than 50 m, corresponding to a delay difference of 0.167 xcexcs (50 m/speed of light)), the receiver will experience a flat fading channel without any frequency diversity in the 4 MHz bandwidth of the signal, due to the coherence bandwidth being approximately 6 MHz {[50 m/(300 Mm/sec)]xe2x88x921≈6 MHz} which totally encompasses the bandwidth of the transmitted signal. Thus, in this case the performance of the single frequency network (SFN) will be poor. Even if the path difference between the two transmit stations is about 100 m, which corresponds to a delay difference of 0.33 xcexcs or channel coherence bandwidth of about 3 MHz, the performance of the system will be poor because of inadequate frequency diversity.
The present invention discloses a method for increasing the performance of an OFDM single frequency network containing multiple transmitters. Increased performance in the OFDM system is accomplished by applying time varying offsets unique to each transmitter on the frequency components of a data signal. Varying the offsets of the frequency components at the individual transmitters decreases the correlation coefficient of the system, effectively reducing the coherence bandwidth. By reducing the coherence bandwidth, flat fading regions due to Rayleigh fading and signal cancellation between transmitters can be reduced, thus providing superior performance.
The method comprises the steps of generating a stream of complex symbols, adding a phase rotation to the complex symbols at individual transmitters where the phase rotation is unique to the individual transmitters, transforming the phase rotated complex symbols from the frequency domain to the time domain, and transmitting the time domain signal from the multiple transmitters.
The broadcast system is comprised of multiple transmitters. Each transmitter receives the same stream of data to be transmitted. Each transmitter has a coder for generating a differential encoded complex symbol at each transmitter, a rotator for adding a phase rotation to the differentially encoded complex symbol, and an inverse fast Fourier transform for transforming the differentially encoded complex symbol from the frequency domain to the time domain.
The present invention can also be incorporated into multiple receivers receiving a signal from a single transmitter or from multiple transmitters.